For example, a gas turbine is typically composed of a compressor, a combustor, and a turbine. The compressor compresses air, taken in through an air inlet, into high-temperature high-pressure compressed air. The combustor supplies fuel to this compressed air and combusts the mixture to produce high-temperature high-pressure combustion gas. The turbine is driven by this combustion gas, and in turn drives a generator coupled coaxially to the turbine.
In this gas turbine, an exhaust system is provided on the downstream side of the turbine. The turbine has a plurality of vanes and a plurality of blades installed inside a turbine casing, alternately in the combustion gas flow direction. An exhaust chamber is installed on the downstream side of the turbine casing through an exhaust casing. This exhaust chamber has an exhaust diffuser that is supported by the exhaust casing. The exhaust diffuser is composed of cylindrical outer diffuser and inner diffuser that are coupled together by strut shields. Struts are each coupled at one end to a bearing that supports a rotor and coupled at the other end to the exhaust casing. Cooling air from the outside is drawn into the gap between the strut and the strut shield to cool the exhaust diffuser.
Japanese Patent Laid-Open No. H06-173712 (JP 06-173712) discloses a perforated plate that is provided at the inlet of an exhaust diffuser cooling system to which a cooling fluid delivered through a guide pipe of a cooling unit is supplied, and that divides a cooling fluid passage of the exhaust diffuser cooling system into a non-heated part for cooling the exhaust diffuser and a heated part for cooling a turbine stage casing. This perforated plate is intended to block radiation heat to eliminate the possibility that the cooling fluid may be overheated by being exposed to heat radiation from the exhaust diffuser and fail to sufficiently cool the turbine stage casing. Part of the cooling fluid supplied to the exhaust diffuser passes through the perforated plate and is supplied to the non-heated part for cooling the exhaust diffuser, and cools the exhaust diffuser before being discharged to the outside.
Japanese Patent Laid-Open No. 2005-083199 (JP 2005-083199) discloses a second cooling system that is defined by a partition wall provided on the outer circumferential side of an inner circumferential casing, and forms an air chamber between the inner circumferential casing and the partition wall.
As described above, the conventional exhaust diffuser is cooled by drawing in cooling air from the outside into the gap between the strut and the strut shield. However, to meet recent demands for higher-power gas turbines, the exhaust gas temperature needs to be raised, which requires further cooling of the exhaust diffuser. In that case, it is conceivable to increase the amount of cooling air drawn from the outside into the gap between the strut and the strut shield. The problem is that this may result in lower turbine efficiency due to lower exhaust gas temperature, as air having cooled the exhaust diffuser is discharged to the exhaust gas flow passage.
The perforated plate disclosed in JP 06-173712 has a structure that allows passage of a cooling fluid, so that the passage formed between the perforated plate and the outer diffuser does not have high cooling efficiency nor good cooling performance. On the other hand, the air chamber disclosed in JP 2005-083199 is not provided on the outer diffuser. Mounting the air chamber disclosed in JP 2005-083199 on the outer diffuser disclosed in JP 06-173712 will pose a challenge of how to support the load that would be placed on the outer diffuser.
Having been devised to solve the above problems, the present invention aims to provide an exhaust system and a gas turbine that can efficiently cool the exhaust diffuser.